An angle detecting apparatus disclosed in JP2007-40850 is used, for example, as a steering angle sensor, a shaft position sensor and the like for an automobile. In the angle detecting apparatus, a magnetic field detecting portion detects magnitudes of components of magnetic fields in two directions in terms of a magnetic field generated around a rotation member. The two directions correspond to a radial direction of the rotation member, and a direction that a north magnetic polar region and a south magnetic polar region are circumferentially arranged, which is orthogonal with respect to the radial direction of the rotation member. A computing means computes a rotation angle of the rotation member based on a ratio of the magnitudes of the components of the magnetic fields in the two directions detected by the magnetic field detecting portion. (Refer to JP2007-40850.)
The magnetic field detecting portion includes, for example, a hall IC as shown in FIG. 2. The magnetic field detecting portion 3 includes a magnetic plate 8, and multiple detecting elements 9 (i.e., a set of detecting elements 9) detecting the magnetic field generated around a rotation member 2. The magnetic plate 8 is disk-shaped. The set of detecting elements 9 (i.e., hall elements) are positioned directly beneath end portions of the magnetic plate 8. The set of detecting elements 9 include a pair of detecting elements 9a, 9b arranged in an X-direction and another pair of detecting elements 9c, 9d arranged in a Z-direction.
For example, as shown in FIG. 19, in the known angle detecting apparatus, the rotation member 2 fitted around a rotation shaft 1 is positioned so that each of north magnetic polar regions 5 and south magnetic polar regions 6 faces radially outwardly. The magnetic field detecting portion 3 is disposed on a substrate 7 which is positioned radially outward of the rotation member 2 and faces each of the north magnetic polar regions 5 and the south magnetic polar regions 6. The magnetic field detecting portion 3 is positioned so that the magnet plate 8 faces parallel to the radial direction of the rotation member 2 for detecting the magnitudes of the components of the magnetic fields in two directions, that is, in the radial direction of the rotation member 2, and in the direction that the north magnetic polar region 5 and the south magnetic polar region 6 are circumferentially arranged.
The detection principle of the magnetic field detecting portion 3 will be described referring to FIGS. 3A, 3B and 19. FIGS. 3A and 3B are sectional views seen from the X-direction and illustrating states of a magnetic flux.
In this example, the radial direction of the rotation member 2 corresponds to the Y-direction, the direction that the north magnetic polar region 5 and the south magnetic polar region 6 are circumferentially arranged corresponds to the X-direction, and a direction orthogonal with respect to the Y-direction and the X-direction corresponds to the Z-direction.
As shown in FIG. 3B, when an external magnetic field is applied in the Y-direction, the magnetic flux is bent by the magnetic plate 8 and components of the magnetic fields are generated on the pair of detecting elements 9c, 9d arranged in the Y-direction, the components of the magnetic fields being perpendicular to the magnetic plate 8, that is, in the Z-direction. At this time, a magnitude of a component of a magnetic field in the Z-direction is proportional to a magnitude of the applied external magnetic field. In addition, directions are opposite between the component of the magnetic field generated on the detecting element 9c and the component of the magnetic field generated on the detecting element 9d. Consequently, a component of a magnetic field that is proportional to the magnitude of the applied external magnetic field is detected by calculating a difference between an output voltage of the detecting element 9c and an output voltage of the detecting element 9d. When an external magnetic field is applied in the X-direction, components of magnetic fields in the Z-direction, that is, perpendicular to the magnetic plate 8, are generated similarly to when the external magnetic field is applied in the Y-direction. Therefore, the magnetic field detecting portion 3 detects a magnitude of a component of a magnetic field in the X-direction by calculating a difference of output voltages between the pair of detecting elements 9a, 9b arranged in the X-direction.
A possible disturbing external magnetic field is removed in a following manner. As shown in FIG. 3A, when the external disturbing magnetic field is applied in the Z-direction, components of magnetic fields are generated on the pair of detecting elements 9c, 9d arranged in the Y-direction, the components of the magnetic fields being perpendicular to the magnetic plate 8. At this time, directions of the generated components of the magnetic fields are the same between the detecting elements 9c and 9d. Consequently, the disturbing external magnetic field applied in the Z-direction is canceled out by calculating a difference between an output voltage of the detecting element 9c and an output voltage of the detecting element 9d. 
As shown in FIG. 19, in the known angle detecting apparatus, a distance R from a surface of the north magnetic polar region 5 or a surface of the south magnetic polar region 6 to the magnetic field detecting portion 3 is long, the distance R being in the radial direction of the rotation member 2. Due to this, the north magnetic polar region 5 and the south magnetic polar region 6 need to be made of, for example, rare-earth magnet having a high magnetic attraction in order to assure desired magnetic field intensity of the magnetic field applied to the magnetic field detecting portion 3, thereby causing cost increase. However, when the distance R from the surface of the north magnetic polar region 5 or the surface of the south magnetic polar region 6 to the magnetic field detecting portion 3 is reduced, magnetic field intensity applied to the magnetic field detecting portion 3 may become too high. When the magnetic field intensity applied to the magnetic field detecting portion 3 is too high, a magnetic flux on the magnetic plate 8 becomes dense, thereby making the magnetic plate 8 magnetically saturated. When the magnetic plate 8 is magnetically saturated, a magnetic field of which intensity is proportional to the external magnetic field is not generated in the perpendicular direction to the magnetic plate 8. Therefore, when the distance R from the surface of the north magnetic polar region 5 or the surface of the south magnetic polar region 6 to the magnetic field detecting portion 3 is reduced, magnitudes of components of magnetic fields in the two directions may not be detected by the magnetic field detecting portion 3.
In the known angle detecting apparatus shown in FIG. 19, magnetic field intensity applied to the magnetic field detecting portion 3 is not constant and varies according to a rotation angle of the rotation member 2 as shown in FIG. 20. FIG. 20 shows the magnetic field intensity applied to the magnetic field detecting portion 3 as the rotation angle of the rotation member 2 is changed. A state shown in FIGS. 19A and 19B, where a circumferentially central portion of the north magnetic polar region 5 faces the magnetic field detecting portion 3, corresponds to a state where the rotation angle is 0 degrees. The magnetic field intensity applied to the magnetic field detecting portion 3 also varies according to variations of dimensions or characteristics of the magnetic field detecting portion 3. An ideal range of the magnetic field intensity (for example, form 20 mT to 70 mT) is defined for operating the magnetic field detecting portion 3. Therefore, an operating range of the magnetic field detecting portion 3 may not be set to be larger than the defined upper limit. In addition, since the magnetic field intensity applied to the magnetic field detecting portion 3 varies as described above, the operating range of the magnetic field detecting portion 3 may be small. Consequently, the magnetic field detecting portion 3 may be susceptible to effects of an external disturbing magnetic field, leading to reduction of detection accuracy. This requires a countermeasure such as providing a magnetic shield, thereby increasing a cost of the angle detecting apparatus.
A need thus exists for an angle detecting apparatus which is not susceptible to the drawback mentioned above.